An Investigation of the Cubic-Hexagonal Transition in Barium Titanate

Normal perovskite-type barium titanate may be converted to the hexagonal polymorph by firing in a hydrogen atmosphere at 1330°C, 130° lower than that previously reported for firing in air. With suitable additives such as MnCO 3 the conversion begins at temperatures as low as 1100°C. Conversion is inhibited by a number of impurities, especially Si, Fe and Sr, such as are present in barium titanate of commercial quality. The effect of atmospheres of hydrogen and oxygen on the rate of reconversion to the cubic polymorph has also been studied. In hydrogen this reaches a peak rate at about 1200°C, while in oxygen it rises continuously. All the phenomena associated with the transition appear to be compatible with a proposed hypothesis about the dependence of free energy on vacancy concentration. The hexagonal polymorph has a permittivity of about 440 and a temperature coefficient of +0.0028 (deg C) -1 .